The present invention relates to new and useful polymers, and more specifically, to improved nonleachable, optically transparent, homogeneous radiopaque heavy metal-containing polymers, compositions of matter, their method of preparation and articles prepared therefrom.
Translucent polymeric materials, and particularly, acrylic type resins have been widely used for years in both medical and dental applications. In dentistry, for example, resins have been used to produce removable dentures, temporary crown and bridge materials, restorative materials, impression materials, and the like. Polymeric resins also find many applications in medicine, such as surgical and body implants and other prosthetic devices, (e.g., heart valves, blood vessels, etc.). Translucent plastics are also widely used in medical appliances, such as catheters.
The desirability of imparting radiopacity to plastics used in dentistry and medicine has long been recognized. In dentistry for example, it has been difficult to detect secondary cavities or underlying decalcified dentin resulting from the placement of unreinforced direct restorative resins because these materials are relatively radiolucent, and are not opaque to x-rays. Surveys have also shown that dental instruments, materials and nonfixed appliances have fractured and become embedded in soft tissues, ingested or inhaled inadvertently by patients. Although incidents of ingestion or inhalation of dental plastics are relatively rare compared with other foreign objects, the occurrence may result in a severe medical emergency or even death. The potential severity of such an incident makes it imperative to diagnose and remove such foreign bodies rapidly.
In medicine, it would be desirable to monitor the positioning of bone cement used in hip joint replacements without surgical procedure. Similarly, it would be desirable to use x-rays to monitor replacement heart valves, replacement arteries, or the path of catheters traversing blood vessels and organ systems. Hence, there is a need for polymeric materials with increased radiation absorption potentials which also possess the requisite nonleachable properties for safe and acceptable use in dentistry and medicine.
Heavy metal salts, such as, for example, those of bismuth or barium have been used as contrast medium in diagnostic radiography. They have properties which would suggest their suitability for increasing the radiation absorption potential of medical and dental resins. As a result, substantial effort has been made to incorporate barium sulfate and other radiopaque salts, such as, for example, bismuth bromide, bismuth chloride or bismuth subnitrate into polymers to render them opaque to x-rays. However, early radiopaque polymers containing heavy metal salts have not been totally satisfactory.
One type of known heavy metal-containing radiopaque materials are radiopaque glass containing embedded heavy metals. In these materials, the metal is not molecularly bound to the polymer matrix and, therefore, has a tendency to weaken the composite. Moreover, because glass filler based resins lack homogeneity a further weakening of regions in the matrix results. Those regions of a composite having little or no glass are radiolucent. In addition, a light scattering effect is produced by radiopaque glasses which alters optical properties and renders them optically opaque.
Polymers with added inorganic heavy metal salts in an essentially physical mixture, are also known. In these materials the heavy metal is present as fine powders locked in a matrix. Their preparation results in an uneven distribution of the salt, which has an adverse affect on the mechanical properties of the plastic material. The salt tends to gradually leach out of the matrix causing discoloration of the polymer and release of heavy metal toxins. The salt and polymer remain as separate distinct phases in these mixtures producing an opaque, cloudy, light scattering material. Mixing does not impart homogeneity between the salt and polymer.
More recently, heavy metal salts have been complexed with a polymer. Such composites require that the complexing polymer contain at least one monomer which is capable of donating a pair of electrons, i.e., acting as a Lewis base. These materials are, therefore, limited in structure since only polymers containing appropriate interaction sites, especially carbonyl moieties, are useful. These heavy metal-salt-polymer complexes are usually moisture sensitive. That is, an initially clear complex will cloud or turn milky upon exposure to moisture, making them inappropriate for certain applications. These heavy metal salt-polymer complexes may also be heat sensitive, and interfere with room temperature curing accelerators used in dental and other applications.